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In the epigrammatic style he has mastered, Tony Blair, former UK prime minister, says that the ideological axis in the modern world is not left versus right but “open versus closed”. Nowhere has pushed openness to such extremes as London in recent decades.

At the turn of the 1980s, it was depopulating, hidebound and still nursing the bruises of post-imperial decline. By the turn of the millennium, it was the entrepôt of globalisation, teeming with migrants and profiting from international capital flows.

Its restaurants, its skyline, its football clubs, its way of earning a living – almost every aspect of London life has been shaken up by outsiders. New York is an American city with a global dimension. London is a global city that happens to be appended to Britain.

It is an unusual revolution that does not provoke a reactionary movement against it and, sure enough, there are incipient challenges to London’s openness. National restrictions on non-European immigration limit the capital’s access to globetrotting talent.

A thicket of financial regulation is tying up the City. There is a nebulous but insistent feeling that openness has curdled into nihilism, that London has become too hospitable to foreign (especially Russian) money, too tolerant of various strains of extremism, too indifferent to British citizens priced out of their own capital city. The rise of the populist UK Independence party, which does well across the country but not in London, is ultimately a reaction against everything the city embodies.

Politicians would be foolish to indulge this backlash and revert to the closed way. London is not just Britain’s moneymaking machine, its wealth skimmed and funnelled to less productive corners of the country. It is also the last trump card that Britain has to play with in the world. The nation’s armed forces are diminishing, as is its relative economic weight. Its permanent membership of the UN Security Council might turn out to be decidedly impermanent. The field of middle-ranking powers is becoming congested, with countries such as Turkey and Brazil competing for strategic influence in the world.

The one distinctive and globally coveted asset Britain has left is this city, a place that simultaneously hosts, encompasses and fascinates the world. Foreigners who are not interested in Britain are smitten with London. Investors who might prefer French infrastructure or German productivity end up plumping for the ease of doing international business in London.

This openness carries costs, of course, in economic and social disruption. But if the country resents the presence of a cosmopolis that sucks in the world’s money, people and ideas, it should remember what it was like not having these things.

Nostalgia is a kind of amnesia: the romanticisation of old London forgets how shabby and short of opportunity it was, entire neighbourhoods languishing in dilapidation, industries declining and being replaced by nothing in particular.

Where Canary Wharf and City Airport now sit were, as recently as the 1980s, toxic marshes and squalid housing estates. In King’s Cross, where the Eurostar zooms in and out from Paris and a new urban village is taking shape, there was desolation and homelessness. This was closed London, when skyscrapers were forbidden and the business culture was a kind of organised slouch that was yet to be blown apart by the Big Bang. It is bizarre to remember all this and conclude that more has been lost than has been gained.

Nostalgia is a kind of amnesia, the romanticisation of old London forgets how shabby and short of opportunity it was

If London abdicates as the unofficial capital of globalisation, other cities to its east are primed to take advantage. Singapore, Hong Kong, Shanghai, Mumbai, Dubai – one of these could emerge as the nexus of international trade as Asia becomes the piston of the world economy.

If London has an advantage over all of them, it is not policy (you can find much lower taxes elsewhere) or even geography (somewhere along the Pacific Rim might come to be seen as the real centre of the world in this century) but that culture of boundless openness.

This is a city that makes no demands on newcomers beyond observance of the law. There is almost nothing you can do here, nothing you can wear or say, no obscure code of etiquette you can violate, that will mark you as an outsider. It is, in the grating modern argot, a non-judgmental city.

This kind of culture cannot be replicated overnight by another metropolis. It is a strange brew of ancient British tolerance and hyper-modern liberalism. As the reaction against it builds, we should rally to its defence.